1. Field of the Invention
The present invention relates to a process for the production of a printing plate. More particularly, the present invention relates to a process for the production of a printing plate which can provide a positive print using a positive original, or a positive print using a negative original.
2. Description of the Prior Art
In general, a planographic printing plate comprises an oleophilic ink receptive area where an image area is formed, and a hydrophilic area where a non-image area is formed. In producing such a printing plate, a method comprising rendering a hydrophilic surface oleophilic only at the image areas, a method comprising providing a hydrophilic layer on the surface of an oleophilic material and removing the hydrophilic layer only at the image areas, thereby exposing the oleophic surface, or a like method is known. Hitherto, the so-called positive working light-sensitive printing plate in which a positive duplicate is produced from a positive original without producing an intermediate negative, has been proposed in, for example, U.S. patent application, Ser. No. 397,986, filed Sept. 21, 1964 (corresponding to British Pat. Nos. 1,129,366 and 1,133,856). The method for the production of a printing plate as described in U.S. patent application Ser. No. 397,986 comprises providing a hydrophilic layer containing a nucleus material for the diffusion transfer process on a support having an oleophilic surface, placing a light-sensitive plate coated with a silver halide light-sensitive emulsion on the hydrophilic layer, forming a silver image on the hydrophilic layer by diffusion transfer development, and removing the hydrophilic layer at the corresponding areas by etch-bleach processing utilizing the silver image, thereby exposing the oleophilic support. In accordance with the method as described in U.S. patent application Ser. No. 650,701, filed July 3, 1961 (corresponding to British Pat. No. 1,235,911), a light-sensitive plate produced by coating a direct positive type gelatin-silver halide emulsion having a melting point of about 82.degree. C. on a support having an ink receptive oleophilic surface, for example, polyethylene, is exposed, a silver positive image is formed by development, the emulsion layer at the silver image areas is removed by applying an etch-bleach processing to thereby expose the ink-receptive oleophilic surface, and thus a printing plate is produced. In the method of U.S. Pat. No. 3,326,685, a light-sensitive plate produced by coating a layer containing a tanning developer on a support, coating a silver halide emulsion layer on the layer, and furthermore, coating a fogged silver halide emulsion layer on the silver halide emulsion layer, is used, and development is conducted by treatment with an alkaline solution after exposure. In this case, the light-sensitized emulsion layer is tanning-developed, thereby forming a negative image. On the other hand, at the non-image areas in the unsensitized areas, the upper-most and fogged silver halide emulsion layer is tanning-developed and gelatin at the areas is hardened depending upon the exposure amount, thereby forming an oleophilic image area.
However, printing plates produced by these methods have the following defects. First of all, these printing plates are all produced for the purpose of reproducing a line drawing original, and thus they are disadvantageous in that reproduction of gradation is bad. In general, in the case of a photographic printing plate, it is necessary to change an original having a continuous gradation into a half-tone gradation. In accordance with presently used standard methods, an original having a continuous tone is printed out on a high-contrast lith-type light-sensitive material by covering the original with a screen, lith-type development accompanied with infectuous development is conducted to thereby produce a half-tone negative or half-tone positive comprising a half-tone image of hard dots, the resulting negative or positive is printed out on a suitable printing plate, for example, a pre-sensitized plate, and development processing is conducted, thereby forming a printing plate.
However, in the case of resolving directly into half-tone gradation by covering an original with a screen, where generally used developers are employed, only silver half-tone dot images having soft dots are obtained. As can be predicted from the principle of projection, the light pattern of the screen mesh is cone-shaped, and at the center, light is the strongest, and at the periphery, light is the weakest, and the light strength varies continuously between them. A photographic light-sensitive material has the ability to show an intermediate density in addition to a maximum density and a minimum density (fog) required for forming a half-tone dot and thus, as can be naturally predicted, a silver image is formed according to the light pattern. Thus, since the density is decreased as the half-tone dot advances from the center to the periphery, density gradient areas naturally occur which is not advantageous in plate making. As a result, only a half-tone dot image of soft dots is obtained. Therefore, with the above described light-sensitive materials and using the above described method, it is impossible to produce a photographic half-tone dot plate. A printing plate produced by direct-screening from an original having a continuous gradation has extremely poor tone-reproduction and furthermore extremely poor printing stability. Thus, with these known methods, it has not been possible to obtain a duplicate having good continuous gradation. In accordance with the method as disclosed in U.S. Pat. No. 3,650,742, a light-sensitive plate produced by coating a silver halide emulsion on an aluminum sheet having an anodic oxidation film on the surface thereof is exposed, a silver negative image is formed by conducting lith-development, the silver image areas are etched by applying etch-bleach processing to thereby uncover the hydrophilic anodic oxidation film, and furthermore, the emulsion layer at the non-image areas is hardened by heating the plate to thereby form an ink-receptive oleophilic gelatin image. As a result, a printing plate is produced in which the image areas are hydrophilic and the non-image areas are oleophilic. Since conventional lith-type light-sensitive materials and lith-type processing are employed in the present invention, a silver half-tone image obtained by direct screening has extremely hard dots free from fringe. However, this method has a second defect in the subsequent etch-bleach processing. That is, the half-tone dot area is increased by the etch-bleach processing, thereby causing discontinuous increases of the density, and furthermore, the non-image area is also side-etched at the shadow area and the half-tone dot is made larger, and thus the tone reproduction is extremely bad.
As is apparent from the above explanation, a printing plate produced by direct screening from an original having a continuous gradation in accordance with known methods has extremely poor tone reproduction. As a matter of fact, it has been impossible to produce a duplicate having a good continuous gradation by known methods.